This invention relates to a pneumatic tire displaying improved drainage characteristics when the vehicle travels on a wet road surface.
In general, when a vehicle runs on a wet road surface, a coefficient of friction between a tire and the road surface decreases. Especially, when the depth of the water on a wet road surface is large, or, when the vehicle runs on a wet road surface at a high speed, a water layer is formed between a tire surface and the road surface, so that the coefficient of friction decreases greatly. This condition is called hydroplaning. When hydroplaning occurs, the steering of the vehicle becomes impossible, and an accident is liable to occur. In order to prevent the occurrence of this hydroplaning, it is necessary to improve the drainage characteristics of a tire.
In order to improve the drainage characteristics, a pneumatic tire which has heretofore been employed is provided in a tread surface thereof with V-shaped auxiliary grooves extending in the same manner in one circumferential direction of the tread surface with the opposed parts of these grooves arranged in substantially parallel with each other in addition to a main groove extending in the circumferential direction of the tread surface, the apexes of these V-shaped auxiliary grooves being positioned substantially on the center (equator) of the width of the tire. However, in a tire having such a so-called directional tread pattern, the drainage characteristics lower when the vehicle turns to cause the ground contacting center of the tread surface to be displaced toward a shoulder as a load moves toward the shoulder.
Japanese patent application Kokai publication No. 02-175304 proposes a pneumatic tire having a special tread pattern, i.e., provided with a plurality of main grooves, and V-shaped auxiliary grooves the apexes of which are distributed in regions extending from the equator of the tire to left and right to a width accounting for 5-25% of the width of a ground contacting portion of a tread surface, in such a manner that the numbers of the apexes on these left and right regions become substantially equal to each other. This tire enables the drainage efficiency during the turning of the vehicle to be improved as compared with a tire in which the apexes of V-shaped auxiliary grooves arranged on the equator.
Although the drainage efficiency of the pneumatic tire of Japanese patent application Kokai publication No. 02-175304 during the turning of a vehicle is superior, the drainage efficiency thereof during a straight travel of a vehicle still remains at the level of that of a conventional pneumatic tire in which the apexes of the V-shaped auxiliary grooves are positioned on the center of the width of the tire, and is not always satisfactorily high.